Sealing the area to prevent air leakage between the inside of the home and the outside environment is important. Failure to do so can create drafts and problems with temperature control, but sealing too much might lead to ventilation problems. One of the best ways to seal a crawl space is to hire a licensed contractor who will know which materials work best to prevent air leakage. Typically, a contractor will detect the air leaks, assess the home's ventilation needs for air quality and use caulk, weatherstripping and other materials to cover and fill cracks between construction materials.
Moisture control is also important. If your home is too humid, it could develop mold problems or wood rot. Hire a contractor to install a vapor barrier or a vapor diffusion retarder, which slows the rate at which moisture can pass through an area. The amount of moisture control you need depends on your region's climate and your home's ventilation, so use an expert who knows how to analyze the air quality in your home correctly.
Once the air leakage and moisture is under control, you can insulate your crawl space. Some building codes require special ventilation in crawl spaces to prevent moisture buildup, so consult your local building department to determine what types of insulation are suitable for your home's ventilation setup. Typically, if your crawl space is unventilated, you should insulate the walls of the foundation rather than the subfloor, according to the U.S. Department of Energy. This method requires less insulation and protects pipes and duct work from freezing problems. Disadvantages exist as well, such as increased risk of damage by rodents, pests or water, so ask an expert if installing insulation across the entire subfloor would be preferable.
Unless you're a qualified expert, you should hire a professional to achieve proper insulation in your crawl space. Besides the risks of high-energy usage and uncomfortable indoor air quality, insulating your home improperly can lead to mold and decay. If radon is a problem in your region, your crawl space's insulation also must incorporate radon resistance. Controlling for all these factors requires experience, proper testing equipment and knowledge of suitable insulating materials and methods.